1. Field of the Invention
This invention relates to a fuel vapor purging system for an automotive vehicle or others.
2. Description of the Prior Art
In automotive vehicles, fuel vapor tends to occur in a fuel tank. For automotive emission control, it is necessary to prevent such fuel vapor from leaking to the atmosphere. Some fuel vapor purging systems for automotive vehicles include a charcoal canister which absorbs fuel vapor transmitted from a fuel tank. During certain conditions, the fuel vapor is drawn from the canister into an air intake section of an automotive engine.
Japanese published unexamined utility model application 60-127465 discloses a charcoal canister which has a plurality of absorption chambers filled with activated charcoal. In Japanese application 60-127465, the charcoal canister has a vapor inlet successively followed by the absorption chambers, and the vapor inlet is connected to a fuel tank. In addition, the canister has a purge outlet connected to an engine air induction passage via a check valve.
In Japanese application 60-127465, during a fuel vapor absorbing process, fuel vapor flows from the tank into the canister via the vapor inlet. Then, the fuel vapor successively flows through the absorption chambers while being absorbed by the charcoal therein. The absorption chambers are arranged into a configuration which provides a long distance of a path of the flow of the fuel vapor in the charcoal to attain an adequate efficiency of the absorption of the fuel vapor by the charcoal.
In Japanese application 60-127465, during a vapor separating process, the check valve is opened so that the purge outlet of the canister is moved into communication with the engine air induction passage. Thus, the interior of the canister is subjected to a negative pressure, that is, an engine air induction vacuum. As a result of the vacuum, the fuel vapor is separated from the charcoal in the canister and is then purged via the purge outlet into the engine air induction passage. The canister also has an air inlet. During the vapor separating process, fresh air is introduced into the canister via an air inlet and is then drawn into the engine air induction passage together with the fuel vapor. The introduction of fresh air into the canister reduces a pressure loss in the canister and promotes the separation of the fuel vapor from the charcoal.
It is now assumed that an automotive vehicle equipped with such a fuel vapor purging system remains left without activating an engine for several days. During the daytime, the atmospheric temperature is usually high so that fuel evaporates in the fuel tank. The resultant fuel vapor is absorbed by the charcoal in the canister. During the night, the atmospheric temperature is usually low so that a vacuum occurs in the fuel tank. As a result of the vacuum, the fuel vapor is separated from the charcoal in the canister and is then returned via the vapor inlet to the fuel tank. In addition, air is introduced into the canister via the air inlet and is then moved toward the fuel tank together with the fuel vapor. Thus, the absorption of the fuel vapor by the canister and the return of the fuel vapor from the canister to the fuel tank are alternately repeated several times. In cases where the amount of the fuel vapor returned to the fuel tank is small, the charcoal in the canister tends to be saturated and hence the fuel vapor overflows from the canister via the air inlet during the daytime. Accordingly, a great amount of the fuel vapor returned to the fuel tank is desirable for automotive emission control.